The present invention relates to a method for sheet forming a gypsum hardened body which is composed mainly of gypsum dihydrate, slag and calcium hydroxide and is remarkably waterproof.
A method for sheet forming a gypsum hardened body which is composed mainly of gypsum dihydrate and slag mixed with fibrous reinforcing agents has been disclosed in U.S. Pat. No. 4,146,402. However, as the gypsum hardened body of the above disclosed method has a water absorptivity going up to about 60% in the 24 hour water absorption test, it has proved to be unsatisfactorily durable due to its deterioration caused by permeation, freezing and melting of water during its use as an outdoor board having a small bulk specific gravity. Especially, as the above disclosed hardened body contains a large quantity of gypsum dihydrate, the gypsum dihydrate can be dissolved by permeation of water. Therefore, it is more deteriorated during its use as an outdoor construction material than other gypsum dihydrate free hardened bodies, which comprise for example, portland cement, slag cement and mortar.
As a counterplan therefor, a method of applying a resin coating on the surface of a hardened body manufactured in factories has been carried out. However, even in this method, as the cut end of a hardened body cutted at construction fields absorbs water, it is required to apply a resin coating on the cut end thereof again.
Therefore, it is desirable to apply a waterproofing treatment also to the whole inner part of the hardened body for preventing water absorption through the cut end thereof.
Upon such demand, the present inventors began to investigate a method in which a waterproofing treatment of the inner part of a hardened body can be carried out simultaneously with manufacturing of a board. That is because it is difficult to apply conventional waterproofing treatment methods as such to a gypsum hardened body which is manufactured by sheet forming and is intended to be used mainly as an outdoor board. The above conventional waterproofing treatment methods are methods which use various kinds of waterproofing agents for construction materials on the markent such as paraffin emulsion, sodium methlsiliconate, silicone emulsion, metal soap and silicone powder.
A method for manufacturing a board by sheet forming is a method in which raw materials are made to be dispersed uniformly into a large quantity of water to be slurried and the thus obtained uniform slurry is molded by a sheet forming machine mainly into a board under filtration of the large quantity of water contained in the slurry. The molded board has two uniform plane surfaces. As a sheet forming machine, HATSCHEK machine is generally used.
In the method for sheet forming a gypsum hardened body composed mainly of gypsum dihydrate and slag, the use of the above conventional waterproofing agents proved to be ineffective as follows: